Acoustic wave diagnostic apparatus and control method thereof

ABSTRACT

Periodic displacement occurs in body tissue due to heartbeat. A peak level D of the movement distance of the body tissue is detected (Step  21 ), and a heartbeat cycle T is calculated from a frequency spectrum (Steps  22  and  23 ). By dividing twice the peak level D by the heartbeat cycle T, the moving velocity of the body tissue in a unit heartbeat cycle is calculated (Step  24 ). By dividing the moving velocity by a frame rate r, an average movement distance of the body tissue between frames is calculated (Step  25 ). In a case where the average movement distance is smaller than a predetermined threshold value, a time interval between the frames used for the calculation of the movement distance is extended (being Step  26  NO, Step  27 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.15/941,178 filed Mar. 30, 2018, which is a Continuation of PCTInternational Application No. PCT/JP2016/077765 filed on Sep. 21, 2016,which claims priority under 35 U.S.C. § 119(a) to Japanese PatentApplication No. 2015-196268 filed on Oct. 1, 2015. Each of the aboveapplications is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a diagnostic apparatus using, acousticwaves, for example, ultrasonic waves, and a control method thereof.

2. Description of the Related Art

Various diagnostic apparatuses using living body information for medicaldiagnosis have been developed (JP2014-36778A, JP3268396B, JP2010-22418A,JP2012-75950A, JP2008-167838A, JP2006-181058A, JP2003-284718A, andJP2003-250767A). In recent years, information (strain) on the hardnessor softness of body tissue has been measured using ultrasonic waves, andelastography using this information for medical diagnosis has also beenknown. JP2014-36778A describes generating an elastic image representingthe hardness (strain) of body tissue, using the movement of the bodytissue resulting from heartbeat. As described in JP3268396B, in general,the movement of the body tissue is measured with the movement distanceor displacement between respective points on two time-series tomograms.JP2010-22418A describes adjusting a frame rate with reference to acorrespondence table according to the number of heartbeats per minute.

SUMMARY OF THE INVENTION

The rate or magnitude of the heartbeat varies depending on subjects(patients). For example, in the case of a subject with extremely slowheartbeat, the movement distance or displacement between the twotime-series tomograms becomes small. As a result, accurate strain cannotbe measured under the influence of flickering (noise) of signals. Inthis case, an elastic image representing the finally obtained strainalso becomes inaccurate. Even in a case where the time interval betweenthe two tomograms is excessively narrow (the frame rate is extremelyhigh) with respect to the rate of the heartbeat, the movement distanceor displacement between the two time-series tomograms becomes small. Asa result, this case is also affected by the flickering (noise) of thesignals.

An object of the invention is to maintain a high-accuracy elastic imagewith little noise or with no noise irrespective of the rate of theheartbeat of each of the subjects.

An acoustic wave diagnostic apparatus according to the inventioncomprises an acquisition device (acquisition means) for acquiringacoustic wave frame data at a predetermined frame rate, using anacoustic wave echo signal representing an acoustic wave echo reflectedfrom body tissue of a subject; a movement distance calculating device(movement distance calculating means) for calculating a movementdistance of the body tissue, using a pair of acoustic wave frame dataitems; an elastic image generating device (elastic image generatingmeans) for generating an elastic image representing strain calculatedfrom the movement distance of the body tissue calculated by the movementdistance calculating device; a heartbeat cycle calculating device(heartbeat cycle calculating means) for calculating a heartbeat cycle ofthe subject; an average movement distance calculating device (averagemovement distance calculating means) for calculating an average movementdistance of the body tissue between the acoustic wave frame data itemsin the heartbeat cycle of the subject, using the movement distance ofthe body tissue calculated from each of a plurality of pairs of acousticwave frame data items by the movement distance calculating device andthe heartbeat cycle of the subject calculated by the heartbeat cyclecalculating device; and an adjusting device (adjusting means) forextending a time interval between the acoustic wave frame data itemsused for the calculation of the movement distance of the body tissue andthe generation of the elastic image in a case where the average movementdistance calculated by the average movement distance calculating deviceis smaller than a predetermined threshold value.

The invention also provides a control method suitable for the acousticwave diagnostic apparatus. That is, this method comprises acquiringacoustic wave frame data at a predetermined frame rate by an acquisitiondevice on the basis of an acoustic wave echo signal representing anacoustic wave echo reflected from body tissue of a subject; calculatinga movement distance of the body tissue by a movement distancecalculating device, using a pair of acoustic wave frame data items;generating, by an elastic image generating device, an elastic imagerepresenting strain calculated from the movement distance of the bodytissue calculated by the movement distance calculating device;calculating a heartbeat cycle of the subject by a heartbeat cyclecalculating device; calculating an average movement distance of the bodytissue between the acoustic wave frame data items in the heartbeat cycleof the subject by an average movement distance calculating device, usingthe movement distance of the body tissue calculated from each of aplurality of pairs of acoustic wave frame data items by the movementdistance calculating device and the heartbeat cycle of the subjectcalculated by the heartbeat cycle calculating device; and extending atime interval between the acoustic wave frame data items used for thecalculation of the movement distance of the body tissue and thegeneration of the elastic image by an adjusting device in a case wherethe average movement distance calculated by the average movementdistance calculating device is smaller than a predetermined thresholdvalue.

As the time interval between the acoustic wave frame data items used forthe calculation of the movement distance of the body tissue and thegeneration of the elastic image is extended, the average movementdistance of the body tissue calculated by the average movement distancecalculating device becomes large. In one embodiment, the time intervalbetween the acoustic wave frame data items can be extended such that theaverage movement distance coincides with the threshold value (may notcoincide perfectly and may coincide approximately).

Preferably, the acoustic wave diagnostic apparatus further comprises aregion-of-interest setting device (region-of-interest setting means) forsetting a portion of an acoustic wave image represented on the basis ofthe acoustic wave echo signal as a region of interest. The movementdistance calculating device calculates the movement distance of the bodytissue included in the region of interest set by the region-of-interestsetting device.

In one aspect, the average movement distance calculating device obtainsa heartbeat average velocity of the subject, using the movement distanceof the body tissue calculated from each of the plurality of pairs ofacoustic wave frame data items and the heartbeat cycle of the subjectcalculated by the heartbeat cycle calculating device, and calculates theaverage movement distance of the body tissue by dividing the heartbeataverage velocity by the frame rate.

In another aspect, the plurality of pairs of acoustic wave frame dataitems are obtained over one heartbeat cycle of the subject, and theaverage movement distance calculating device calculates the averagemovement distance of the body tissue by taking an average of movementdistances of the body tissue calculated from the plurality of pairs ofacoustic wave frame data items, respectively.

Preferably, a movement distance immediately after the start of heartbeatand immediately before the end of the heartbeat among the movementdistances of the body tissue calculated from the plurality of pairs ofacoustic wave frame data items, respectively, over the one heartbeatcycle is excluded from the calculation of the average movement distance.

In one aspect, the adjusting device extends the time interval bylowering the frame rate of the acquisition device.

In another aspect, the adjusting device extends the time interval bythinning out and using the acoustic wave frame data used for thecalculation of the movement distance of the body tissue from theacoustic wave frame data acquired by the acquisition device.

Advantage of the Invention

According to this invention, in a case where the average movementdistance of the body tissue is smaller than the predetermined thresholdvalue, the time interval between the acoustic wave frame data items usedfor the calculation of the movement distance of the body tissue and thegeneration of an elastic image is extended. Thus, the movement distanceof the body tissue between the acoustic wave frame data items can beincreased. As the movement distance of the body tissue between theacoustic wave frame data items is increased, it is possible to preventor reduce the appearance of noise in the elastic image, and it ispossible to maintain a high-accuracy elastic image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the inside of a living body wherediseased tissue is present.

FIG. 2 illustrates the positions of peripheral tissue and the diseasedtissue before being pressed, the positions of the peripheral tissue andthe diseased tissue after being pressed in a case where movementdistance is large, and the positions of the peripheral tissue and thediseased tissue after being pressed in a case where the movementdistance is small.

FIG. 3 is a graph illustrating the displacement of the peripheral tissueand the diseased tissue before and after being pressed.

FIG. 4 is a graph illustrating the strain of the peripheral tissue andthe diseased tissue before and after being pressed.

FIG. 5 schematically illustrates an elastic image regarding a subjectwith fast heartbeat.

FIG. 6 schematically illustrates an elastic image regarding a subjectwith slow heartbeat.

FIG. 7 is a block diagram illustrating an overall configuration of theultrasonic diagnostic apparatus.

FIG. 8 is a flowchart illustrating a flow of processing for generatingan elastic image.

FIG. 9 is a flowchart illustrating a flow of the processing forgenerating the elastic image.

FIG. 10 is a graph illustrating the time variations of cumulative valuesof reference movement distances.

FIG. 11 illustrates frequency spectra corresponding to heartbeats.

FIG. 12 illustrates an aspect in which the frame rate of the ultrasonicframe data is lowered.

FIG. 13 illustrates an aspect in which the ultrasonic frame data isthinned out.

FIG. 14 illustrates movement distance Δd calculated for a pair ofultrasonic frame data items in a plurality of ultrasonic frame dataitems acquired within a heartbeat cycle T.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, elastic images will be schematically described with reference toFIG. 1 to FIG. 6 . FIG. 1 schematically illustrates the inside of aliving body where hard diseased tissue exists so as to be surrounded bysoft peripheral tissue (normal body tissue). Reference sign 31 of FIG. 2schematically represents the peripheral tissue and the diseased tissuebefore being pressed (at the time of relaxation) caused by heartbeat ina cross-sectional position along line II-II of FIG. 1 , that is, passingthrough the diseased tissue. Reference signs 32 and 33 of FIG. 2designate the peripheral tissue and the diseased tissue after beingpressed (at the time of compression), reference sign 32 schematicallydesignates a state where the peripheral tissue and the diseased tissueare greatly displaced due to heartbeat, and reference sign 33schematically designates a state where the peripheral tissue and thediseased tissue are slightly displaced (a state in the middle of beingdisplaced) due to heartbeat. In order to make this easily understood,FIG. 2 illustrates that a left end of the peripheral tissue is fixed (nodisplacement).

FIG. 3 is a graph graphically illustrating the displacement of theperipheral tissue and the diseased tissue before and after beingpressed, which are respectively designated by reference signs 31 and 32and reference signs 31 and 33 of FIG. 2 . FIG. 4 is obtained bydifferentiating the displacement illustrated in FIG. 3 . In FIGS. 3 and4 , graphs (relationships between reference signs 31 and 32 of FIG. 2 )regarding a subject with relatively fast heartbeat (the movementdistance of the body tissue is relatively large) are illustrated bythick solid lines, and graphs (relationships between reference signs 31and 33 of FIG. 2 ) regarding a subject with relatively slow heartbeat(the movement distance of the body tissue is relatively small) areillustrated by thin solid lines.

FIG. 5 schematically illustrates an elastic image 41 regarding thesubject with fast heartbeat, and FIG. 6 schematically illustrates anelastic image 42 regarding the subject with slow heartbeat. The elasticimages represent the hardness (the magnitude of strain) of the bodytissue. As an example of the methods of displaying the elastic images,the hardness of the body tissue can be represented by hues according tothe degree of the hardness. In the elastic images, hard body tissue,that is, body tissue with small strain, is expressed in blue. Soft bodytissue, that is, body tissue with great strain, is expressed in red.Body tissue with intermediate hardness is expressed in green. In FIGS. 5and 6 , a body tissue range displayed in blue is indicated by “B”, abody tissue range displayed in red is indicated by “R”, and a bodytissue range displayed in green is indicated by “G”. Here, assignment ofthe hues according to the hardness as an example of the methods fordisplaying the elastic images is not limited to the above, and can bearbitrarily set. Additionally, as another example of the methods fordisplaying the elastic images, the elastic images (the hardness of thebody tissue) may be expressed by brightness, instead of the hues (colordifferences). That is, using specific hues, such as red, blue, andgrayscales, for example, the hard body tissue, that is, the body tissuewith small strain can be darkly expressed, and the soft body tissue,that is, the body tissue with great strain can be brightly expressed.

The elastic images are obtained by visibly expressing the strainrepresenting the hardness of the body tissue calculated as describedabove. The strain is calculated by differentiating the movement distance(displacement) of the body tissue resulting from heartbeat(relationships between FIG. 3 and FIG. 4 ).

The tissue movement distance is calculated using a pair of ultrasonicframe data items at different acquisition times, which are obtained byultrasonic measurement. The time interval between the pair of ultrasonicframe data items used for the calculation of the tissue movementdistance is assumed to be constant. Regarding the subject with fastheartbeat, a substantially total range of tissue movement is acquired inthe pair of ultrasonic frame data items at different acquisition times(the relationship between reference signs 31 and 32 of FIG. 2 ). Thatis, regarding the subject with fast heartbeat, a large movement distanceis calculated using the pair of ultrasonic frame data items having apredetermined time interval, which are used for calculating the movementdistance.

Next, a subject with extremely slow heartbeat will be considered. In acase where a pair of ultrasonic frame data items having the same timeinterval as the predetermined time interval at which the large movementdistance can be calculated regarding the subject with fast heartbeat isused, a partial movement distance of tissue movement may not be acquiredin the total range of the tissue movement regarding a subject withextremely slow heartbeat (the relationship between reference signs 31and 33 of FIG. 2 ). In this case, a small movement distance iscalculated.

Regarding the subject with fast heartbeat (the subject in which thelarge tissue movement distance is calculated between the pair ofultrasonic frame data items), referring to the graph of the thick solidlines of FIG. 3 , a large difference is caused between the displacementof the soft peripheral tissue and the displacement of the hard diseasedtissue (the inclination of the graph is completely different). Referringto the graph of the thick solid lines of FIG. 4 , regarding the subjectwith fast heartbeat, the strain calculated by differentiating thedisplacement can be calculated to have a large value for the softperipheral tissue and can be calculated to have a small value for thehard diseased tissue. Referring to FIG. 5 , the blue B representing thatthe diseased tissue is hard (strain is small) is illustrated in theelastic image regarding the subject with fast heartbeat. A color (here,green G) showing that the peripheral tissue is softer than the diseasedtissue is illustrated. The existence of the hard body tissue (diseasedtissue) can be visually recognized.

In contrast, regarding the subject with extremely slow heartbeat (thesubject in which the small tissue movement distance is calculatedbetween the pair of ultrasonic frame data items), referring to the graphof the thin lines of FIG. 3 , a difference is not recognized between thedisplacement of the peripheral tissue and the displacement of thediseased tissue and a noise signal may appear. In a case where this isdifferentiated, referring to the graph of the thin lines of FIG. 4 , avalue (noise) that does not originate from the difference between thehardness of diseased tissue, and the hardness of peripheral tissue willbe calculated as the great strain.

Referring to FIG. 6 , regarding the subject with slow heartbeat, a hue(here, red (R): color representing softness) affected by noise willappear in the elastic image. That is, the hardness of the body tissuecannot be accurately represented on the elastic image.

As illustrated in detail below, the ultrasonic diagnostic apparatus ofthis example keeps the movement distance of the body tissue betweenultrasonic frame data items equal to or larger than a certain value andthereby prevents noise (FIG. 6 ) from appearing in the elastic image ormakes noise less likely to appear in the elastic image, by calculatingthe average movement distance (an average value of the tissue movementdistance between ultrasonic frame data items generated by theperiodically repeated heartbeat) of the body tissue due to heartbeat andextending the time interval between the ultrasonic frame data items usedto calculate the movement distance and used to generate an elasticimage, in a case where the average movement distance of the body tissuecaused by heartbeat is smaller than a predetermined threshold value(prescribed value).

In this example, the ultrasonic diagnostic apparatus using ultrasonicwaves will be described. The present invention is not limited to theultrasonic waves, and acoustic waves of audible frequencies may be usedas long as a suitable frequency is selected in accordance with objectsto be detected, measurement conditions, or the like.

FIG. 7 illustrates a block diagram illustrating an overall configurationof an ultrasonic diagnostic apparatus 1. The ultrasonic diagnosticapparatus 1 includes an ultrasonic probe 2, a transmission/receptionbeam former 3, an echo data processing device 4, an image control device5, a display device 6, an operating device 7, a control device 8, and astorage device 9.

The operation of ultrasonic diagnostic apparatus 1 is entirelycontrolled by the control device 8. A control program, a hue conversionlook-up table, frame data, and the like for controlling the variousdevices (to be described in detail below) that constitute the ultrasonicdiagnostic apparatus 1 are stored in the storage device 9 connected tothe control device 8. An operator's instruction, a value to be set oradjusted, and the like are input from the operating device 7.

The ultrasonic probe 2 is pressed against the body surface of thesubject (patient). The ultrasonic probe 2 transmits an ultrasonic beamtoward the subject, receives an ultrasonic echo reflected from the bodytissue within the subject, and outputs an ultrasonic echo signalrepresenting the ultrasonic echo. The ultrasonic probe 2 having anarbitrary shape, such as a convex type, a sector type, or a linear type,can be used.

The transmission/reception beam former 3 drives the ultrasonic probe 2under predetermined scanning conditions, and performs scanning with anultrasonic beam. An arbitrary scanning method, such as sector scanning,offset sector scanning, or linear scanning, can be adopted.Additionally, the transmission/reception beam former 3 performspredetermined signal processing, such as phasing addition processing, onthe ultrasonic echo signal from the ultrasonic probe 2, and generatesultrasonic frame data (tomographic echo data) corresponding to onescanning surface (one tomogram) of the body tissue. The generatedultrasonic frame data is sequentially stored in the storage device 9.The ultrasonic frame data is generated over time depending on apredetermined frame rate (the number of ultrasonic frame data itemsgenerated per unit time). The frame rate of the ultrasonic frame datacan be changed by changing the transmission rate of the ultrasonic beamtransmitted by the ultrasonic probe 2.

Next, the ultrasonic frame data is input to the echo data processingdevice 4. The echo data processing device 4 includes a B-mode datagenerating unit 4A, a movement distance measuring unit 4B, and a straincalculating unit 4C.

The B-mode data generating unit 4A performs logarithmic compressionprocessing, envelope detection processing, or the like on the ultrasonicframe data to generate B-mode data.

The movement distance measuring unit 4B calculates the followingmovement distance, using the ultrasonic frame data. That is, themovement distance of each unit region (each pixel) is calculated on thebasis of a pair of ultrasonic frame data items among a plurality ofultrasonic frame data items at different acquisition times, which aresequentially stored in the storage device 9, and the movement distanceframe data is generated. Thereafter, a reference movement distance thatis a representative value of the movement distance is calculated fromthe movement distance of each unit region. As the reference movementdistance to be calculated, for example, an average value of movementdistances for respective unit regions in a region of interest (ROI) setin a portion of an ultrasound image can be used. However, the movementdistance of a specific unit region, such as the center of a frame, maybe used as the reference movement distance, or a plurality of unitregions may be specified and an average value of the movement distanceof the plurality of specified unit regions may be used. Moreover, aplurality of ROIs may be set, an average value of movement distances forrespective unit regions in each ROI may be calculated regarding each ofthe set ROIs, the average of the average values calculated regarding theplurality of ROIs, respectively, may be further calculated. In addition,even in a case where the ROI is within a predetermined range, the ROImay be within a range specified using the operating device 7 by a user.Processing using the reference movement distance (representative value)will be described below.

The strain calculating unit 4C obtains the strain of each unit region onthe basis of the movement distance frame data generated by theabove-described movement distance measuring unit 4B, and generatesstrain frame data. The strain frame data is generated (calculated) bydifferentiating the movement distance frame data.

The B-mode data output from the B-mode data generating unit 4A of theecho data processing device 4 and the strain frame data output from thestrain calculating unit 4C are input to the image control device 5. Theimage control device 5 includes a B-mode image data generating unit 5A,an elastic image data generating unit 5B, and an image display controlunit 5C.

The B-mode image data generating unit 5A performs scanning conversion,using a scan converter on the B-mode data, and generates two-dimensionaltomogram data (B-mode image data) suitable for the display in thedisplay device 6. In the B-mode image data, signal intensity isrepresented by luminance. The B-mode image data has, for example,information representing the luminance of 256 gradations for each pixel.

The elastic image data generating unit 5B executes the processing ofgenerating elastic image data in color representing the strain of eachunit region in the strain frame data by a hue (a color difference)according to the magnitude of the strain, that is, making the straininto a hue. In the elastic image represented by the elastic image data,the magnitude of strain is represented by differences in hue. Acorrespondence relationship between the magnitude of strain and the huesis based on the hue conversion look-up table (not illustrated) stored inadvance in the storage device 9.

The image display control unit 5C synthesizes the B-mode image data andthe elastic image data, and generates synthesized image data. By thesynthesized image data being transmitted to the display device 6, asynthesized ultrasound image in which a B-mode image and the elasticimage are synthesized is displayed on a display screen of the displaydevice 6. Indeed, the B-mode image and the elastic image may bedisplayed on the display screen without being synthesized.

FIGS. 8 and 9 are flowcharts illustrating the processing of generatingthe elastic image data in the ultrasonic diagnostic apparatus 1.

Regarding a subject having a body tissue physically displaced due toheartbeat, ultrasonic frame data corresponding to one scanning surface(one tomogram) is continuously acquired (measurement of ultrasonicwaves) (Step 11). The ultrasonic frame data is acquired at apredetermined frame rate, and is sequentially recorded in the storagedevice 9.

A pair of ultrasonic frame data items in the ultrasonic frame datasequentially recorded in the storage device 9 is used to calculate themovement distance of each unit region in the echo data processing device4 and to generate movement distance frame data. Moreover, theabove-described reference movement distance (representative value) iscalculated (Step 12).

Data representing the reference movement distance for a certain time isstored in the storage device 9. FIG. 10 is a graph illustrating the timevariations of cumulative values of reference movement distances for 5seconds.

The time variation (dashed line A) of a cumulative value of thereference movement distance for 5 seconds in a subject A having aheartbeat of 60 times/min and the time variation (solid line B) of acumulative value of the reference movement distance for 5 seconds in asubject B having a heartbeat of 48 times/min are illustrated in FIG. 10. The dashed line A of FIG. 10 records about five heartbeats (5 cycles).The solid line B records four heartbeats (4 cycles).

The data representing the cumulative values of the reference movementdistances illustrated in FIG. 10 is used to calculate an averagemovement distance d of the body tissue between the pair of frame dataitems in the heartbeat equivalent to 1 cycle (hereinafter referred to asa heartbeat average movement distance d) and to perform the processingof adjusting the time interval of the frame data on the basis of thecalculated heartbeat average movement distance d (Step 13). Thecalculation of the heartbeat average movement distance d and theadjustment of the time interval of the frame data are performed by thecontrol device 8 along the following flow (FIG. 9 ).

First, the peak levels (peak movement distances) D (mm) of thecumulative values of the reference movement distances are detected(refer to FIG. 10 ) (Step 21). Here, in order to make this easilyunderstood, the peak movement distances D of the subject A and thesubject B are assumed to be the same.

Next, the data representing the cumulative values of the referencemovement distances is subjected to fast Fourier transform (FFT), andfrequency spectra are obtained (Step 22). FIG. 11 illustrates afrequency spectrum (dashed line) corresponding to the heartbeat of thesubject A illustrated in FIG. 10 , and a frequency spectrum (solid line)corresponding to the heartbeat of the subject B. For example, in a casewhere the frame rate of the ultrasonic frame data is assumed to be 30[fps], the number of data items for 5 seconds is 30×5=150. Since it isdesirable to have data items in the power of 2 in the FFT calculation,frequency spectra can be obtained using the most recent 128 data items.

Moreover, the peak levels of the frequency spectra are searched for,frequencies corresponding to the peak levels are determined as heartbeatfrequencies, and heartbeat cycles (s) are obtained by calculating theinverses of the heartbeat frequencies [Hz] (Step 23). For example, in acase where the heartbeat frequency of the subject A is f_(A)=1.0 [Hz],the heartbeat cycle T_(A) of the subject A is obtained as T_(A)=1.0 [s].For example, in a case where the heartbeat frequency of the subject B isf_(B)=0.8 [Hz], the heartbeat cycle T_(B) of the subject B is obtainedas T_(B)=1.25 [s].

The body tissue repeats compression and relaxation due to heartbeat. Itis possible that twice of the peak movement distance D is the totalmovement distance of the body tissue in the heartbeat equivalent to 1cycle. Hence, the average velocity v [mm/s] of the heartbeat can beobtained by dividing the total movement distance 2D by the heartbeatcycle (2D/T) (Step 24). For example, regarding the above-describedsubject A of the heartbeat cycle T_(A)=1.0 [s], in a case where the peakmovement distance is D=10 [mm], the heartbeat average velocity v_(A) of2D/T_(A)=20/1=20 [mm/s] is calculated. Regarding the subject B of theheartbeat cycle T_(B)=1.25 [s], the heartbeat average velocity v_(B) of2D/T_(B)=20/1.25=16 [mm/s] is calculated.

The frame rate of ultrasonic diagnostic apparatus 1 is defined as r[fps]. 1/r is equivalent to a time interval [s] between successiveframes. The heartbeat average movement distance d is calculated bydividing the heartbeat average velocity by the frame rate r (multiplyingthe time interval 1/r between frames) (Step 25). In a case where theframe rate is set to 30 [fps], the heartbeat average movement distanced_(A) of v_(A)/r=20/30=0.67 [mm] is calculated regarding the subject A.The heartbeat average movement distance d_(B) of 16/30=0.53 [mm] iscalculated regarding the subject B.

Special processing is not performed in a case where the heartbeataverage movement distance d obtained as described above is equal to orgreater than the predetermined threshold value (prescribed value) (YESin Step 26). On the other hand, in a case where the heartbeat averagemovement distance d is smaller than the threshold value, it isdetermined that the heartbeat is slow (or the frame rate is too highwith respect to the rate of the heartbeat), and the processing ofextending the time interval between the ultrasonic frame data items usedfor the calculation (Step 12) of the movement distance is performed (NOin Step 26, Step 27). The heartbeat average movement distance d can beincreased by extending the time interval between the ultrasonic framedata items.

In a case where the heartbeat average movement distance d becomessmaller than the threshold value, noise appearing in an elastic imageposes a problem. The maximum value of the heartbeat average movementdistance d at which noise (refer to FIG. 6 ) appearing in an elasticimage poses a problem is investigated, and the value is stored in thestorage device 9 as the threshold value. Since the threshold valuechanges depending on the properties of an ultrasonic probe to be used,the threshold value may be stored in advance in association with anindividual ultrasonic probe. However, the ultrasonic diagnosticapparatus 1 may enable its user to input the threshold value using theoperating device 7 or to adjust the threshold value, using a slider barbetween a predetermined maximum value and a predetermined minimum value.

As illustrated in FIG. 12 , the time interval between the ultrasonicframe data used for the calculation of the movement distance can beextended by lowering the frame rate of the ultrasonic frame data. Theframe rate of the ultrasonic frame data can be lowered byfeedback-controlling the ultrasonic probe 2 and thetransmission/reception beam former 3 by the control device 8 andchanging the transmission rate of the ultrasonic beam.

The above time interval is adjusted such that the heartbeat averagemovement distance d has the threshold value. For example, in a casewhere the threshold value is 0.53 [mm], the calculated heartbeat averagemovement distance d is 0.47 [mm] (the heartbeat average velocity is 14[mm/s] and the frame rate is 30 [fps]), the frame rate may be loweredsuch that the frame rate of the ultrasonic frame data has 14/0.53=26.42[fps]. The heartbeat average movement distance d (14/26.42) can be madeto substantially coincide with the threshold value.

As illustrated in FIG. 13 , the time interval between the ultrasonicframe data items used for the calculation of the movement distance maybe extended not by changing the frame rate of the ultrasonic frame databut by, for example, alternately skipping (thinning out) the ultrasonicframe data items used for the calculation of the movement distance. Thisthinning-out processing is executed by feedback-controlling the echodata processing device 4 by the control device 8. Referring to FIG. 13 ,supposing the tissue movement distance is calculated using frame dataT−1 and frame data T, and the frame data T and frame data T+1, which arecontinuously acquired before the adjustment of the time interval, thetissue movement distance is calculated from the frame data T−1 and theframe data T+1 after the adjustment of the time interval by the framedata T being thinned out. As a result, for example, in a case where thethreshold value is 0.67 [mm] (20 [mm/s] in a case where the thresholdvalue is expressed by heartbeat average velocity), and the calculatedheartbeat average movement distance d is 0.33 [mm] (10 [mm/s] in a casewhere the threshold value is expressed by the heartbeat averagevelocity), the heartbeat average movement distance d can be made tocoincide with the threshold value without lowering the frame rate of theultrasonic frame data by keeping the frame rate of the ultrasonic framedata at 30 [fps] without being changed and alternately skipping theultrasonic frame data used for the calculation of the movement distanceas described above. Additionally, as described above, since theultrasonic frame data is also used for the generation of the B-modeimage, it is not necessary to lower the frame rate in the B-mode image.

In a case where the adjustment of the time interval of the ultrasonicframe data is performed as described above, the ultrasonic frame dataacquired depending on the time interval after the adjustment is used,and the movement distance frame data is generated.

Referring back to FIG. 8 , the strain of each unit region (each pixel)is calculated by differentiating the movement distance frame data in theecho data processing device 4, and the strain frame data is generated(Step 14). Next, in the image control device 5, strain frame data isconverted into hues, using the hue conversion look-up table stored inadvance in the storage device 9, and the elastic image data is generatedby the hue conversion (Step 15). An elastic image in which the magnitudeof strain is expressed by differences in hue is displayed on the displaydevice 6 under the control of the image control device 5 (Step 16).

In this way, by adjusting the time interval between the ultrasonic framedata items that are used for the calculation of the movement distanceand for the generation of the elastic image data such that the heartbeataverage movement distance d reaches the predetermined threshold value,the movement of the body tissue exceeding the threshold value isdetected. As a result, an elastic image affected slightly by noise (FIG.5 ) or not affected by noise can be displayed.

In the above-described example, an example in which the average velocityv of the heartbeat is calculated and the heartbeat average movementdistance d is calculated by dividing the average velocity by the framerate r has been described. However, as illustrated in FIG. 14 , theheartbeat average movement distance d can also be calculated byobtaining all movement distances Δd calculated for each pair ofultrasonic frame data items in a plurality of ultrasonic frame dataitems of the frame rate r acquired within the heartbeat cycle T afterthe heartbeat cycle T [s] is obtained and by taking the average of themovement distances (the average of integrated values). The heartbeataverage movement distance d can be calculated by the following Formula.

$\begin{matrix}{d = \frac{\int_{T}{❘{\Delta d}❘}}{rT}} & \lbrack {{Formula}1} \rbrack\end{matrix}$

Instead of using all the movement distances Δd included in the heartbeatcycle T for the calculation of the heartbeat average movement distanced, the heartbeat average movement distance d may be calculated byexcluding one or more movement distances Δd immediately after the startof the heartbeat and immediately before the end of the heartbeat amongall the movement distances Δd included in the heartbeat cycle T, fromcalculation of the heartbeat average movement distance d. A situation inwhich the heartbeat average movement distance d may fluctuate vigorouslydue to extreme noise can be avoided. In that case, it is preferable tocalculate the heartbeat average movement distance d by excludingmovement distances Δd that belong to top N % and bottom N % (N is a realnumber of 10 or less) among all the movement distances Δd.

EXPLANATION OF REFERENCES

-   -   1: ultrasonic diagnostic apparatus (acoustic wave diagnostic        apparatus)    -   2: ultrasonic probe (an acquisition device)    -   3: transmission/reception beam former (an acquisition device)    -   4: echo data processing device (a movement distance calculating        device)    -   5: image control device (an elastic image generating device)    -   6: display device    -   7: operating device (a region-of-interest setting device)    -   8: control device (a heartbeat cycle calculating device, an        average movement distance calculating device, an adjusting        device)    -   9: storage device

What is claimed is:
 1. An acoustic wave diagnostic apparatus comprising:a controller configured to: acquire acoustic wave frame data at apredetermined frame rate, using an acoustic wave echo signalrepresenting an acoustic wave echo reflected from body tissue of asubject; calculate a movement distance of the body tissue, using a pairof acoustic wave frame data items; generate an elastic imagerepresenting strain calculated from the movement distance of the bodytissue that is calculated; calculate a heartbeat cycle by inverting afrequency peak from a frequency spectra corresponding to the movementdistance of the body tissue; calculate an average movement distance ofthe body tissue between the acoustic wave frame data items in theheartbeat cycle of the subject, using a movement distance of the bodytissue determined from each of a plurality of pairs of acoustic waveframe data items and the calculated heartbeat cycle; and extend a timeinterval between the acoustic wave frame data items to calculate asubsequent movement distance of the body tissue from elastic imagesbased on thresholding, wherein the time interval extension is executedby thinning out the acoustic wave frame data while maintaining thepredetermined frame rate in a case where the average movement distanceis smaller than a predetermined threshold, and wherein the subsequentmovement distance is calculated based on the time interval that isextended.
 2. A method of controlling an acoustic wave diagnosticapparatus comprising: acquiring acoustic wave frame data at apredetermined frame rate, using an acoustic wave echo signalrepresenting an acoustic wave echo reflected from body tissue of asubject; calculating a movement distance of the body tissue, using apair of acoustic wave frame data items; generating an elastic imagerepresenting strain calculated from the movement distance of the bodytissue that is calculated; calculating a heartbeat cycle by inverting afrequency peak from a frequency spectra corresponding to the movementdistance of the body tissue; calculating an average movement distance ofthe body tissue between the acoustic wave frame data items in theheartbeat cycle of the subject, using a movement distance of the bodytissue determined from each of a plurality of pairs of acoustic waveframe data items and the calculated heartbeat cycle; and extending atime interval between the acoustic wave frame data items to calculate asubsequent movement distance of the body tissue from elastic imagesbased on thresholding, wherein the time interval extension is executedby thinning out the acoustic wave frame data while maintaining thepredetermined frame rate in a case where the average movement distanceis smaller than a predetermined threshold, and wherein the subsequentmovement distance is calculated based on the time interval that isextended.